Commonly known as ‘Australian Mint Bush’
As you would expect from Australian plants very few are hardy enough to grow outside in the UK. The exception is the alpine P. cuneata which looks as though it should be hardier and generally is! P. lasianthos will survive outside in the mildest areas and even grows into a tall shrub in Ventnor Botanic Gardens in the Isle of Wight. P. rotundifolia can just about survive a mild winter with us but this is usually a disaster.
Once you accept that prostanthera are greenhouse plants requiring a frost free environment in the winter you can go on to enjoy the many and varied forms of this attractive plant. Most species give off a minty scent, some even without being touched (P. rotundifolia and P. ovalifolium especially).
Prostanthera come in a wide variety of species. The most commonly grown and easiest as small shrubs are P. sieberi (lilac purple), P. rotundifolia (lilac purple to purple), P. lasianthos (white) and P. walteri (bluish green). Prostanthera start flowering once the weather warms up and they start to grow. The first panicles or racemes of flowers appear on the new growth in April or May, sometimes earlier. When grown in large containers or open beds they will flower on progressively through the summer. You can assist this process by giving them a light haircut after flowering to induce secondary new growth and more flowers.
Apart from the dwarf species (P. walteri, P. phylicifolia) prostanthera will grow as large and as quickly as you let them or can feed them on to achieving. They are greedy plants.
The taller growing varieties quickly get a bit leggy and unsightly after three to four years. Once tall and woody they do not respond to hard pruning. It is therefore sensible to take new growth cuttings from time to time to start the plants off again. The cuttings root very quickly.
As with many greenhouse plants white fly and red spider mite can cause problems. If you can, stand the plants outside to reduce the problem or use an insecticide.